1. Technical Field
The present invention relates to wireless communication, and more particularly, a method of managing non-acknowledgement (NAK) responses sent by a receiver in a wireless communication system.
2. Description of Related Art
In wireless communication systems, an air interface is used for the exchange of information between a mobile station and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels. In wireless transmission, a channel is time varying due to fading, mobility, and so on. More specifically, channel quality may be affected by factors such as distance between the mobile and base station, speed of the mobile station, interference, and the like. Given the limited resources (e.g., bandwidth) of wireless transmission as well as the large number of mobile stations supported by a base station at any given time, and therefore competing for those limited resources, it is important to maximize throughput of a wireless communication system.
Protocols such as the Hybrid-Automated Repeat reQuest (H-ARQ) have been introduced to improve the overall system capacity. FIG. 1 illustrates a portion of the layered structure for a transmitter 10 (e.g., such as in a base station) and a receiver 20 (e.g., such as in a mobile station) in a wireless communication system as set forth in third generation wireless standards such as CDMA-2000 employing H-ARQ. In CDMA-2000, for example, the H-ARQ may be called Asynchronous and Adaptive Incremental Redundancy (AAIR). As shown, a Radio Link Protocol (RLP) 12 in the medium access control (MAC) 14 layer of the transmitter sends data packets for transmission to a physical layer 16. The data packets include a sequence number identifying their order in a sequence of transmitted data packets. The physical layer 16 turbo encodes the data packet and transmits a portion of the encoded data packet—this portion being referred to as a sub-packet—over a H-ARQ channel assigned for communication between the transmitter 10 and the receiver 20. The physical layer 26 of the receiver 20 receives the sub-packet and attempts to decode the sub-packet to obtain the entire data packet. If successful, the data packet is sent to the RLP 22 in the MAC 24 of the receiver 20. The physical layer 26 also sends an acknowledgement response (ACK) to the transmitter 10 for a properly received data packet. If the physical layer 26 is unable to decode the sub-packet, then the physical layer 26 sends a non-acknowledgement (NAK) response to the transmitter 10.
The physical layer 16 of the transmitter 10 expects to receive an ACK or NAK response two time slots after sending the sub-packet. In CDMA-2000, for example, a time slot is 1.25 ms. If an ACK response is received from the physical layer 26, the physical layer 16 encodes and transmits another data packet to any scheduled user in the system. If no response or a NAK response is received from the physical layer 26, the physical layer 16 transmits the next sub-packet to the same user. Standards such as CDMA-2000 establish a maximum number of sub-packet transmissions for each packet. When the maximum number of sub-packet transmissions is reached, the physical layer 16 flushes the data packet in its buffer. The physical layer 16 then encodes and transmits another data packet for a user, which is scheduled by the transmitter 10.
In the system described above, it is possible that the RLP 22 of the receiver 20 receives data packets having sequence numbers 1 and 3, respectively, before receiving the data packet of sequence number 2. For example, the three data packets may be sent over three different H-ARQ channels to the receiver 20 and it may take more sub-packet transmissions for proper receipt of data packet 2 as compared to that of data packets 1 and 3. When this happens, the RLP 22 judges the data packet of sequence number 2 as missing and issues a NAK response, which is sent to and received by the RLP 12 of the transmitter 10. In response to the NAK response, the RLP 12 reschedules transmission of the data packet having sequence number 2. However, the data packet having sequence number 2 may eventually be properly received as a result of later sub-packet transmissions; thus, rendering the rescheduling moot. Even if this occurs, the rescheduling continues, and resources and capacity are wasted.